[ticket/13110] Add core event to the page_footer() function

Add core event to the page_footer() in includes/functions.php to allow
extensions handling overall page output before its displaying.

PHPBB3-13110
Merge pull request #3000 from rxu/ticket/13110

[ticket/13110] Add core event to the page_footer() function
Merge branch 'develop-ascraeus' into develop